The present invention relates to a process and a device for detecting the lift-off of an aircraft in the course of take-off.
It is known that the take-off procedure for an aircraft essentially comprises a phase of rolling along the ground followed by a lift-off phase. In the course of the rolling phase, the speed, the longitudinal attitude and the lift of the aircraft increase progressively, thereby partly unloading the landing gear of said aircraft, compressed by the mass of the aircraft. However, when the aircraft, at the end of the rolling phase, begins its lift-off phase, that is to say leaves the ground, said landing gear is completely unloaded of the mass of the aircraft, so that it relaxes significantly.
Also, it is easy to detect the lift-off of an aircraft in the course of take-off by measuring the elongation of said landing gear or else the crushing stresses to which it is subjected.
However, especially on account of the hydraulic damping of said landing gear, a device for detecting the lift-off of an aircraft, based on observation of said gear, cannot provide a response which is fast enough to be usable in a reliable manner in the course of the take-off procedure.
The main object of the present invention is to remedy this drawback and to make it possible to obtain in real time the information that the wheels of an aircraft taking off are leaving the ground.
Accordingly, according to the invention, the process for detecting the lift-off of an aircraft in the course of take-off, is noteworthy in that:
in the course of the take-off procedure, both during the phase of rolling along the ground and during the lift-off phase, the quantity A=K.h+xcex8 is calculated, in which:
xcex8 represents the longitudinal attitude of the aircraft,
h represents the height, with respect to the ground, of the zone of the tail of said aircraft which would touch the ground if the longitudinal attitude xcex8 were to take too high a value during take-off, and
K is a constant equal to the inverse of the longitudinal distance separating said tail zone from the articulations of the landing gear of said aircraft thereon;
the value of said quantity calculated during at least part of said rolling phase is stored;
said stored value is compared with the actual value in the course of calculation, of said quantity; and
said lift-off is detected by the fact that said actual value becomes greater than said stored value.
Thus, through the implementation of the process in accordance with the present invention, it is possible to obtain the lift-off detection information in real time. The process of the present invention is based on the finding by the applicant that the quantity A remains almost constant during said rolling phase, but grows rapidly and steeply when the wheels of the aircraft leave the ground, at the start of the lift-off of said aircraft.
It is therefore seen that, according to the present invention, the constant value of the quantity A during the rolling phase, frozen and stored, serves as reference for the calculations. It is with this constant frozen value that the growing value of the quantity A will be compared, after the lift-off of the aircraft.
In order to avoid the nuisance effects of measurement noise the value of said quantity calculated during at least part of the rolling phase is averaged before storage. Such averaging can be performed throughout the duration of the rolling phase. However, it may advantageously begin from the moment at which the aircraft reaches while rolling a predetermined speed, for example that from which the aircraft""s on-board computers provide credible information.
Preferably, although it can be carried out at any moment of the rolling phase subsequent to the averaging, the storage of said value of the quantity calculated during at least part of said rolling phase is performed, at the latest, at the instant at which said aircraft reaches while rolling the speed of rotation, that is to say the speed at which the pilot of the aircraft begins to pull back on the stick with a view to lift-off.
Furthermore, for safety, it is preferable to add a safety margin to said stored value, before comparing the latter with said actual value in the course of calculation.
For the implementation of the process according to the invention, there is a provided a device comprising:
means for measuring the longitudinal attitude xcex8 of said aircraft;
means for determining the height h, with respect to the ground, of the zone of the tail of said aircraft which would touch the ground if the longitudinal attitude xcex8 were to take too high a value during take-off;
means for calculating in the course of the take-off procedure, both during the phase of rolling along the ground and during the lift-off phase, the quantity A=K.h+xcex8, in which K is a constant equal to the inverse of the longitudinal distance separating said tail zone from the articulations of the landing gear of said aircraft thereon;
means of storage of the value of said quantity calculated during at least part of said rolling phase; and
means of comparison of said stored value with the actual value, in the course of calculation, of said quantity, said means of comparison delivering at their output a comparison signal which takes one or the other of two distinct values, such that:
the first of these values, which is representative of the phase of rolling of the aircraft along the ground, results from the fact that said stored value and said actual value are at least approximately equal;
the second of these values, which is representative of the lift-off of said aircraft, results from the fact that said actual value has become greater than said stored value.
It is then advantageous for said device to comprise:
means for averaging by filtering, disposed between said means of calculation and said means of storage; and
an adder intended to add said safety margin to said stored value and disposed between said means of storage and said means of comparison.
It will readily be understood that, in order to utilize said comparison signal, the device according to the present invention can comprise means of utilization receiving said comparison signal and generating, from the values of the latter, limit values of longitudinal attitude for said aircraft. In this case, the device in accordance with the present invention can constitute an aid to the take-off of the aircraft by making it possible to prevent the tail thereof from touching the ground in the course of the take-off procedure.
It is known in fact that, during take-off and under cetain conditions, the tail of an aircraft may be apt to strike the ground. Also, thought has already been given to protecting the tail of the aircraft against damage which may result therefrom, for example by installing a protective skid, and also to warning the pilot of the proximity of the ground.
For example, documents U.S. Pat. No. 3,493,082 and EP 0 319 051 describe protection tailskids, able to absorb the energy of any shock so as to protect the fuselage from damage. Such skids consist of passive mechanical devices, which of course serve only in the case of actual touching of the ground by the tail. For their part, documents U.S. Pat. No. 3,979,717 and FR 2 547 275 describe alert systems warning the pilot of the proximity of the ground, said pilot then having to act rapidly to avoid the tail touching.
Depending on the nature of his means of utilization, the present invention can provide the pilot with an indication, audible and/or visual, delimiting an advised take-off domain in which there is no possibility of the tail touching. Moreover, if said means of utilization consist of flight computers of the aircraft, they can automatically manage the dynamics of the aircraft in the take-off phase, by utilizing said comparison signal. Thus, within the framework of the present invention, it is therefore not a matter of intervening in the event of the tailskid touching (as in the documents U.S. Pat. No. 3,493,082 and EP 0 319 051), nor of warning the pilot that his aircraft is in a critical position with respect to the ground (as in the documents U.S. Pat. No. 3,979,717 and FR 2 547 275).
The implementation of the device of the invention as a take-off aid is simple because said comparison signal is representative of the take-off phase in the course of execution and since it can be used to anticipate a possible touching of the ground by the tail and indicate to the pilot or to on-board computers a longitudinal attitude limit not to be exceeded, appropriate to the take-off phase in the course of implementation.
In the usual case where said aircraft is equipped with at least one monitoring screen indicating the longitudinal attitudes of said aircraft, an exemplary embodiment of said means of utilization of the device in accordance with the present invention can comprise means for consecutively displaying, on said monitoring screen during take-off:
firstly a first limit value of longitudinal attitude not to be exceeded for said aircraft during the rolling phase, said first limit value being associated with said first value of the comparison signal; then,
a second limit value of longitudinal attitude not to be exceeded for said aircraft during the lift-off of the latter, said second limit value being greater than said first limit value and being associated with said second value of the comparison signal.
Of course, to permit the toggling of the display on the monitoring screen from said first limit value to said second limit value, it is necessary to make certain that particular conditions specific to the take-off of the aircraft all hold. Also, according to another feature of the present invention, there are provided means permitting the replacement, on said monitoring screen of the display of said first limit value by the display of said second limit value only if a take-off is actually about to occur.
To determine whether a take-off is actually about to occur, it is possible, for example, to detect the position of the throttle so as to verify that it is in the xe2x80x9ctake-offxe2x80x9d position.
Furthermore, in order to fix the take-off domain in which the device in accordance with the present invention is active, it is possible to provide means permitting the display of said second limit value only so long as said current instantaneous calculated value of said quantity A is less than a predetermined value, representative of the fact that the aircraft is clear of the ground, take-off having been completed, and/or only so long as the landing gear of the aircraft is not fully relaxed. Specifically, once clear of the ground, the longitudinal attitude indication is no longer required by the pilot. It could even be an annoyance to the pilot. Moreover, the monitoring screens of an aircraft control station being very dense in terms of information, it is preferable not to maintain the display of said second limit value, while it is no longer useful. It is therefore desirable to remove the latter when any risk of the tail touching is eliminated.
Preferably, the information respectively representative of the fact that a take-off is about to occur, that the lift-off of the aircraft has taken place and that the aircraft is clear of the ground are applied through an AND-type logic gate. Thus, when at least one of these three pieces of information does not exist, the display of said second threshold value does not take place.
When the conditions permitting the display of said second threshold value as replacement for said first all hold, the transition is preferably progressive so as to take account of the rotational dynamics of the aircraft. It may for example be done continuously in a time corresponding to a nominal rate of rotation (variation of longitudinal attitude) for a take-off, for example 2 seconds.